More ambient creatures in the world. Ones that either add sounds, or visuals. Examples would be birds, insects, etc.

In terms of birds, you could have Seagulls in Ramshackle Reef adding to the ambience of pirates with their cries and circling overhead, the barn owls in the Farmlands at night hooting away, and smaller birds in Windless Woods tweeting.

They do not have to be actual NPCs for performance reasons, with the insects most likely being able to be an FX.

(Will link any ideas that suggest ambient creatures in the comments)